Pacemaker cells in the atrioventricular junction (AVJ) frequently become the dominant pacemaker of the heart when the sinoatrial node (SAN) is suppressed or when the AVJ automaticity is sufficiently enhanced. This region contains a diverse population of cells which have various degrees of automaticity and properties of conduction which respond differently to autnomic stimulation. The objective of this proposal is to determine the extent of sympathetic and vagal control in modulation AVJ rhythm using an anesthetized canine preparation. Since the timing of a train of vagal stimuli within the cardiac cycle determines to a great extent the slowing of the AVJ rhythm similar to that of SAN rhythm, a comparison of these 2 rhythms in response to the train of stimuli will be made under a variety of experimental settings. Next, the site of pacemaker in the AVJ will be determined prior to and during constant sympathetic stimulation. The site of pacemaker activity will be determined with a multiple electrode plate using the spread of excitation as an indication of the site. The shifting of pacemaker site may explain the oscillatory rhythm of the AVJ following sympathetic stimulation. Finally concurrent sympathetic and vagal stimulation during AVJ rhythm will be applied to determine possible shifts in the pacemaker site. These shifts may explain the moderate autonomic interaction despite extensive autonomic innervation of this region.